Troy weight is a system of units of mass customarily used for precious metals and gemstones. One troy ounce (abbreviated "oz" or "ozt") equals 7001311034768000000♠31.1034768grams exactly (or about 1.0971 oz. avoirdupois).[1] There are only 12 troy ounces per troy pound, rather than the 16 ounces per pound found in the more common avoirdupois system. However, the avoirdupois pound has 7000 grains whereas the troy pound has only 5760 grains (i.e. 12 × 480 grains). Both systems use the same grain defined by the international yard and pound agreement of 1959 as exactly 6998647989100000000♠0.06479891 grams. Therefore, the troy ounce is 480 grains (31.10 g), compared with the avoirdupois ounce, which is 437.5 grains (28.35 g). So the troy ounce is about 10% heavier (ratio 192/175) than the avoirdupois ounce, which is used for everyday purposes in the United States. Although troy ounces are still used to weigh gold, silver, and gemstones, troy weight is no longer used in most other applications. One troy ounce of gold is denoted with the ISO 4217 currency code XAU, while one troy ounce of silver is denoted as XAG.[2]

Troy weight probably takes its name from the French market town of Troyes in France where English merchants traded at least as early as the early 9th century.[3][4] The name "troy" is first attested in 1390, describing the weight of a platter, in an account of the travels in Europe of the Earl of Derby.[3][5]

Charles Moore Watson (1844–1916) proposes an alternative etymology: The Assize of Weights and Measures (also known as Tractatus de Ponderibus et Mensuris), one of the statutes of uncertain date from the reign of either Henry III or Edward I, thus before 1307, specifies "troni ponderacionem"—which the Public Record Commissioners translate as "troy weight". The word "troni" refers to markets. Watson finds the dialect word "troi", meaning a balance in Wright's Dialect Dictionary. Troy weight referred to the tower system; the earliest reference to the modern troy weights is in 1414.[6][7]

Many aspects of the troy weight system were indirectly derived from the Roman monetary system. The Romans used bronze bars of varying weights as currency. An aes grave ("heavy bronze") weighed 1 pound. One twelfth of an aes grave was called an uncia, or in English an "ounce". Before the adoption of the metric system, many systems of troy weights were in use in various parts of Europe, among them Holland troy, Paris troy, etc.[8] Their values varied from one another by up to several percentage points. Troy weights were first used in England in the 15th century, and were made official for gold and silver in 1527.[9] The British Imperial system of weights and measures (also known as Imperial units) was established in 1824, prior to which the troy weight system was a subset of pre-Imperial English units.

The troy ounce in use today is essentially the same as the British Imperial troy ounce (1824–1971), adopted as an official weight standard for United States coinage by Act of Congress on May 19, 1828.[10] The British Imperial troy ounce (known more commonly simply as the imperial troy ounce) was based on, and virtually identical with, the pre-1824 British troy ounce and the pre-1707 English troy ounce. (1824 was the year the British Imperial system of weights and measures was adopted, 1707 was the year of the Act of Union which created the Kingdom of Great Britain.) The English troy ounce was officially adopted for coinage in 1527. Troy ounces have been used in England since about 1400. Before that, various sorts of troy ounces were in use on the continent.[11]

The only troy weight in widespread use today is the British Imperial troy ounce and its American counterpart. Both are currently based on a grain of 0.06479891 gram (exact, by definition), with 480 grains to a troy ounce (compared with 4371⁄2 grains for an ounce avoirdupois).

The British Empire abolished the 12-ounce troy pound in the 19th century, though it has been retained (although rarely used) in the American system.

The origin of the troy weight system is unknown. Though the name probably comes from the Champagne fairs at Troyes, in northeastern France,[13] the units themselves are probably of more northern origin[citation needed]. English troy weights were apparently derived from the nearly identical troy weight system of Bremen. (The Bremen troy ounce had a mass of 480.8 British Imperial grains.)[11]

An alternate suggestion is that the weights come from the Muslim domains by way of the Gold Dirhem (47.966 British Imperial grains), in the manner that King Offa's weights were derived from the silver Dirhem (about 45.0 British grains).

According to Watson, troy to a dialect word troi (balance). Then troy weight is a style of weighing, like auncel or bismar weights, or other kindred methods. The troy weight then refers to weighing of small precious or potent goods, such as bullion and medicines.[6]

The pennyweight symbol is dwt. There are 24 grains in 1 dwt, and 20 dwt in one troy ounce. Because there were 12 troy ounces in the old troy pound, there would have been 240 pennyweights to the pound—the basis of the fact that the old British pound sterling of currency contained 240 pence. (However, prior to 1526, English pound sterling was based on the tower pound, which is 15⁄16 of a troy pound.) The d in dwt stands for denarius, the ancient Roman coin, referred to in the New Testament, that equates loosely to a penny. The symbol d for penny can be recognized in the notation for British pre-decimal pennies, in which pounds, shillings, and pence were indicated using the symbols £, s, and d, respectively. For example, £6 11s 8d indicated six pounds, eleven shillings, and eight pence.

Mint weights, also known as moneyers' weights were legalised by Act of Parliament dated 17 July 1649 entitled An Act touching the monies and coins of England. A grain is 20 mites, a mite is 24 droits, a droit is 20 perits, a perit is 24 blanks.[17][18]

In Scotland, the Incorporation of Goldsmiths of the City of Edinburgh used a system in multiples of sixteen. (See Assay-Master's Accounts, 1681–1702, on loan from the Incorporation to the National Archives of Scotland.) Thus, there were 16 drops to the troy ounce, 16 ounces to the troy pound, and 16 pounds to the troy stone. The Scots had several other ways of measuring precious metals and gems, but this was the common usage for gold and silver.

The Pound was 7716 BI grains, but after the union, rounded to 7680 BI grains. This divides to 16 ounces, each of 16 drops, each of 30 grains. The rounding makes the ounce and grain equal to the English standard.

The Dutch troy system is based on a Mark, of 8 Ounces, the ounce of 20 Engels (pennyweight), the Engel of 32 As. The mark was rated as 3798 Grains, English troy, or 246.084 metric grams. The divisions are identical to the tower system.[19]

King Offa's currency reform replaced the sceat with the silverpenny. This coin was derived from half of a silver dirhem. The weights were then derived by a count of coins, by a mix of Charlemagne and Roman systems. A shilling was set to twelve pence, an ounce to twenty pence, and a pound to twelve ounces or twenty shillings. The penny was quite a lot of money, so weight by coins was not a general practice.

Later kings debased the coin, both in weight and fineness. The original pound divided was the tower pound of 5400 grains, but a later pound of 5760 grains displaced it. Where once 240 pence made a tower pound (and 256 make a troy pound), by the time of the United Kingdom Weights and Measures Act of 1824, a troy pound gives 792 silver pence, still minted as such as Maundy Money.

^ ab"troy, n.2". Oxford English Dictionary. Oxford, England: Oxford University Press. June 2012. The received opinion is that it took its name from a weight used at the fair of Troyes in France

^Partridge, Eric (1958). "Trojan". Origins: A Short Etymological Dictionary of Modern English. London: Routledge and Kegan Paul. p. 3566. OCLC250202885. …the great fairs established for all Europe the weight-standard Troyes, whence…Troy…

^Smith, Adam (1809). An Inquiry Into the Nature and Causes of the Wealth of Nations, Volume 1. 1. London: T. Hamilton. p. 35. The French livre contained, in the time of Charlemagne, a pound, Troyes weight, of silver of a known finess. The fair of Troyes in Champaign was at that time frequented by all the nations of Europe, and the weights and measures of so famous a market were generally known and esteemed.